Alloy and a method of making an alloy

ABSTRACT

A bainitic steel alloy and a method for making such an alloy are disclosed, in which the bainite plates are particularly small, less than 50 nanometres in width. In preferred embodiments of the invention, each bainite plate is surrounded by a film of retained austenite; the level of retained austenite in the alloy is greater than 10%; and the alloy is substantially free of blocky unstable austenite and cementite.

This invention concerns bainitic steel alloys and methods of manufacturing such alloys. It finds particular, though not exclusive, application in the manufacture of alloys suitable for components such as shafts for aerospace applications.

Certain aeronautical components (such as main shafts in gas turbine engines) require particularly high tensile, fatigue and yield strength. If an alloy with high fatigue strength is used, then the shaft walls can be made thinner and so the shaft is lighter. This weight saving can be particularly important in aeronautical applications. It is known to use martensitic steel alloys for such components.

Bainitic steel alloys are also known, such as those described in prior United Kingdom patents GB2352726 and GB2297054. A problem with known bainitic steels is that their heat treatment does not allow complete transformation to bainite and large regions of unstable blocky austenite remain. These are unstable and when the material is stressed they transform to untempered martensite and tend to cause cracking. Known bainitic alloys also form large regions of cementite. This is a brittle phase that can crack when stressed. These problems reduce the mechanical properties of known, bainitic steel alloys, and consequently these alloys are not suitable for use in aeronautical components such as those discussed above.

In general, the design of components such as shafts is constrained by the mechanical limitations of the available alloys. It would therefore be desirable to have a higher strength steel alloy, to permit the design of the components to be further optimised. In particular, it would be desirable to have a steel alloy in which higher strength is achieved without the associated loss of ductility found in known alloys.

The inventors have made the surprising discovery that by careful selection of the composition and processing parameters, it is possible to produce a bainitic steel alloy with tensile, fatigue and yield strengths sufficiently high to permit its use for critical components such as shafts in aircraft engines, but without the compromises in mechanical properties that have previously been associated with high strength bainitic steel alloys.

The invention therefore provides an alloy and a method of making an alloy as set out in the claims.

Several embodiments of the invention will now be described, by way of example only, with reference to FIG. 1, which is a time/temperature/transformation (TTT) diagram for a bainitic steel alloy according to the invention. An alloy according to one aspect of the invention (the “generic composition”) has a composition by weight percent in the ranges 0.6-1.0 wt % carbon, 1-2 wt % silicon, 0.5-2.5 wt % manganese, up to 0.01 wt % phosphorous, up to 0.008 wt % sulphur, up to 1.3 wt % chromium, 0.2-0.4 wt % molybdenum, up to 1.3 wt % nickel, 0.01-0.06 wt % aluminium, up to 4 wt % cobalt, up to 0.07% copper, 0.005-0.06 wt % nitrogen, up to 0.01 wt % niobium, up to 0.07 wt % tin and up to 0.3 wt % vanadium, with the remainder (save for incidental impurities) being iron.

The carbon content of at least 0.6 wt % in this alloy ensures the stabilisation of enough retained austenite to give the alloy the necessary toughness and ductility, and to give it enough hardenability to avoid the pearlite nose. Too much carbon, however, would cause the formation of high temperature cementite, and may also give the alloy too much hardenability so that the bainite reaction takes too long to reach completion.

Silicon has a low solubility in cementite, and thus prevents its formation. At least 1 wt % of silicon is required for this purpose. It has been shown that up to 2 wt % of silicon can be included without degrading the mechanical properties of the resulting alloy.

Manganese is used as a desulphuriser, and at least 0.5 wt % is required for this purpose. Chromium, nickel and manganese all increase hardenability. The total of these three elements should be kept below 5 wt %, otherwise the alloy will have too much hardenability and the bainite reaction will take too long to reach completion to be practicable. The addition of nickel, in place of the chromium commonly used in known bainitic steel alloys, allows the alloy to be austenitised at a lower temperature so that the austenite structure can be refined without the risk of forming cementite. Cementite is undesirable in the alloy because it is associated with brittleness and poor fatigue life. Generally, the lower the austenitisation temperature the smaller the austenite grain size; and the smaller the grain size the better the mechanical properties. Small austenite grains also allow the bainite transformation to occur faster, because there are more nucleation sites for the bainitic ferrite. However, if chromium and nickel are used together this may result in embrittlement.

At least 0.2 wt % of molybdenum is required to prevent phosphorous embrittlement. Too much molybdenum, however, will result in the formation of primary molybdenum carbides which are undesirable.

A certain amount of aluminium and nitrogen is required in the alloy to form aluminium nitride for grain refinement. The aluminium nitride pins the grain boundaries and improves the bainite transformation kinetics, as well as the mechanical properties of the resulting alloy. Aluminium also works in a similar way to silicon to prevent the formation of cementite.

Vanadium can also react to form vanadium carbide, which (like aluminium nitride) can pin grain boundaries. However, if too much vanadium is included large carbides may be precipitated, which can reduce the mechanical properties.

The inclusion of cobalt improves the kinetics of the bainite reaction, without significantly changing the thermodynamics.

In this composition, copper, tin, phosphorous, sulphur and niobium are residual or impurity elements and their levels should be as low as possible. The levels of these elements are generally limited by the steel-making process.

It has been shown that levels of phosphorous in excess of 0.011 wt % tend to cause phosphorous embrittlement, which reduces the strength of the alloy.

Excessive niobium (more than about 0.02%) has been shown to form large niobium carbides, which reduce the strength of the resulting alloy.

A method of making an alloy according to this aspect of the invention will now be described. It will be appreciated that this description is by way of example only and is not to be understood as limiting the scope of protection.

A formulation of the alloy is first processed to produce a raw material melt. Various known melting methods provide a “clean” melting route suitable for producing alloys for critical components, such as those discussed previously. The melting method may, for example, include vacuum melting.

The raw material ingot may be homogenised. Suitable parameters for the homogenisation would be a homogenisation temperature of 1200 degrees Celsius and a homogenisation time period of up to about two days.

The material is re-heated for hot working to a temperature in excess of 1100 degrees Celsius and forged or rolled to final size with a finishing temperature of above 900 degrees Celsius. The forged bar is air cooled to ambient temperature.

The bar is then sub-critically annealed at a temperature of about 750 degrees Celsius for a time in excess of one hour. After annealing, the bar is air cooled to ambient temperature.

The bar is next heated again, to an austenitisation temperature T_(A) of 830 degrees Celsius. The range 10 of possible austenitisation temperatures is shown in FIG. 1. It is desirable to austenitise at as low a temperature as possible (above the austenite start temperature 18) because this will minimise the size of the austenite grains. For an alloy of this composition, the range 10 extends approximately between 780 degrees Celsius and 1000 degrees Celsius.

Bainite nucleates from the austenite grain boundaries, and so a smaller grain size will enable the bainite to transform more quickly. Also, fracture surfaces from tensile and fatigue testing of bainitic alloys show an intergranular appearance, with failure occurring along austenite grain boundaries. This occurs because the grain interiors are relatively strong, so that the weakest points are at the grain boundaries. If the grains are smaller then any cracks have to grow a larger distance.

A further benefit from reducing the austenite grain size is that it reduces the embrittlement effect caused if sulphur and phosphorous segregate to the grain boundaries.

The bar is held at the austenitisation temperature for as long as necessary to form a fully austenitic structure. The time needed for this will of course vary according to the dimensions of the bar.

Once the bar's structure is fully austenitic, it can be cooled. Referring now to FIG. 1, the bold lines show the cooling over time of the bar. Point A is the starting point, at the austenitisation temperature T_(A). The edges of the bar will cool faster than the core, and so the edge temperature follows the line 12, while the core of the bar (which will cool more slowly) follows the line 14. At the point B1 the edges of the bar will have reached the transformation temperature T_(T). The temperature of the bar is controlled so that the edges are maintained at the transformation temperature T_(T) until, at the point B2, the whole bar is at uniform temperature.

An important feature of the cooling curve of FIG. 1 is the avoidance of the pearlite “nose”. The line 20 is the pearlite start line, and 22 the pearlite finish line. The area between these lines indicates the time/temperature region in which pearlite can form in the alloy. Generally, pearlite forms by a eutectoid reaction when austenite is cooled slowly, and so to avoid its formation it is necessary to cool the austenite quickly enough that this reaction cannot take place.

The line 24 is the bainite start line, and the line 26 is the bainite finish line. The area between these lines indicates the time/temperature region in which austenite may be transformed into bainite. Another important feature of the cooling curve is that the bainite reaction must not start before the entire bar reaches the transformation temperature T_(T). This is to minimise residual stresses in the material, and consequent distortion. Therefore, the point 32 must lie to the left of the bainite start line 24.

The bar is then held at temperature T_(T) until it has passed the bainite finish line 26, at point C. Different transformation temperatures change the mechanical properties of the resulting alloy, and require different treatment times. As the transformation temperature is reduced the bainite reaction takes longer to reach the bainite finish line 26, but the microstructure produced is finer. This finer microstructure will tend to increase the alloy's strength but decrease its ductility. The mechanical properties of the alloy can therefore be tailored by a suitable choice of transformation temperature. The required transformation time may be predetermined by hardness testing of a sample bar, because the hardness of the alloy stabilises once transformation is complete. Once this transformation time is known for a given composition and austenitisation temperature, it will be consistent; therefore, periodic checking of the alloy's hardness during routine transformation is not necessary.

During the transformation, the bainite plates form by a shear transformation. The high carbon in the bainitic ferrite is then allowed to diffuse into the retained (untransformed) austenite. Carbon acts as an austenite stabiliser and so the austenite becomes a stable film surrounding the fine bainite plates. Experimental results show that it is necessary that 10% (by volume fraction) of the steel remains austenite so that it provides a continuous film surrounding the bainite plates. This improves the ductility of the alloy.

Once the bainite transformation is complete, at point C, the microstructure is stable and the bar can be furnace cooled to ambient temperature (point D). Even though the cooling line 32 passes through the martensite start line 30, the microstructure remains stable.

A transformation temperature T_(T) in the range between 190 degrees Celsius and 230 degrees Celsius appears to be optimum, but it is possible to perform the transformation at any temperature in the range 16 shown in FIG. 1. For an alloy of this composition, the range 16 extends approximately between 170 degrees Celsius and 300 degrees Celsius. The transformation time will depend on the composition and austenitisation temperature, as described above, and may be between a few hours at a high T_(T) and two weeks or so at a low T_(T).

The alloy may optionally be tempered after the bainite transformation is complete. For an alloy of this composition, the tempering temperature will be between 250 degrees Celsius and 450 degrees Celsius, depending on the size of the bar.

This alloy has better mechanical properties than known bainitic steel alloys for two main reasons: firstly, because the composition prevents the formation of cementite; and secondly, because the bainite transformation is allowed to reach completion isothermally until the bainite finish line. In known bainitic steel alloys, the bainite transformation is typically incomplete, leaving regions of unstable and blocky austenite. When stressed, this can form untempered brittle martensite, which causes cracks.

By performing the austenitisation and bainite transformation at low temperatures, a very fine microstructure is formed. In an alloy according to the invention, made by the method described above, the bainite plates are typically less than 50 nanometres wide, and this gives the alloy high strength. Surrounding each bainite plate is a film of stable austenite, which enhances ductility. As mentioned above, it is important that at least 10% austenite is retained, after the bainite transformation, to achieve the desired ductility.

An alloy according to a further aspect of the invention (the “higher Cr” composition) has a composition by weight percent in the ranges 0.7-0.85 wt % carbon, 1-2 wt % silicon, 0.5-2.5 wt % manganese, up to 0.01 wt % phosphorous, up to 0.008 wt % sulphur, 1-2 wt % chromium, 0.2-0.4 wt % molybdenum, up to 0.08 wt % nickel, 0.01-0.06 wt % aluminium, 1.4-5 wt % cobalt, up to 0.1% copper, 0.005-0.06 wt % nitrogen, up to 0.005 wt % niobium, up to 0.1 wt % tin and 0.1-0.3 wt % vanadium, with the remainder (save for incidental impurities) being iron.

The total of chromium and manganese should be kept below 4 wt %, otherwise (as explained previously) the steel will have too much hardenability and the bainite reaction will take too long to complete. The nickel content of the alloy should be minimised to avoid embrittlement.

The general comments given above regarding the properties of the various alloying elements also apply to the alloy in accordance with this aspect of the invention.

In a particular preferred embodiment of this aspect of the invention the alloy has a composition by weight percent in the ranges 0.75-0.8 wt % carbon, 1.3-1.4 wt % silicon, 1.9-2.2 wt % manganese, 0.005-0.008 wt % phosphorous, up to 0.005 wt % sulphur, 1.2-1.25 wt % chromium, 0.3-0.35 wt % molybdenum, up to 0.06 wt % nickel, 0.02-0.04 wt % aluminium, 1.65-1.75 wt % cobalt, up to 0.05% copper, 0.01-0.04 wt % nitrogen, up to 0.001 wt % niobium, up to 0.05 wt % tin and 0.14-0.16 wt % vanadium, with the remainder (save for incidental impurities) being iron.

The method of making an alloy with either of these compositions will be essentially the same as the method set out in detail above.

A suitable austenitisation temperature for an alloy according to this aspect of the invention will be between 800 degrees Celsius and 1000 degrees Celsius, and preferably between 810 degrees Celsius and 840 degrees Celsius.

A suitable transformation temperature will be between 170 degrees Celsius and 300 degrees Celsius, and preferably between 200 degrees Celsius and 210 degrees Celsius.

If tempering is to be performed, a suitable tempering temperature will be between 250 degrees Celsius and 500 degrees Celsius, and preferably between 250 degrees Celsius and 370 degrees Celsius.

An alloy produced according to this aspect of the invention will typically have an austenite grain diameter of about 10 μm after heat treatment. After transformation, the level of retained austenite is expected to be higher than 10% and preferably within the range 10-20%.

An alloy produced according to this aspect of the invention, with the composition and heat treatment set out above, is expected to have the following mechanical properties:

Tensile strength 2000 MPa at room temperature and 1900 MPa at 350 degrees Celsius.

0.2% proof stress 1650 MPa at room temperature, and 1400 MPa at 350 degrees Celsius.

Elongation to failure 4% at room temperature and 10% at 350 degrees Celsius.

Vickers hardness 660 HV at room temperature.

Fatigue life (stress range 1400 MPa, R=0) greater than 50000 cycles before failure at room temperature, and greater than 40000 cycles under the same conditions at 350 degrees Celsius.

An alloy according to a further aspect of the invention (the “higher Ni” composition) has a composition by weight percent in the ranges 0.7-0.85 wt % carbon, 1.2-2 wt % silicon, 0.5-2.5 wt % manganese, up to 0.01 wt % phosphorous, up to 0.008 wt % sulphur, up to 0.35 wt % chromium, 0.2-0.4 wt % molybdenum, 0.9-2 wt % nickel, 0.01-0.06 wt % aluminium, up to 1.5 wt % cobalt, up to 0.1% copper, 0.005-0.06 wt % nitrogen, up to 0.005 wt % niobium, up to 0.1 wt % tin and up to 0.3 wt % vanadium, with the remainder (save for incidental impurities) being iron.

The total of chromium and manganese should be kept below 4 wt %, otherwise the steel will have too much hardenability and the bainite reaction will take too long to complete. The chromium content of the alloy should be minimised to avoid embrittlement.

The general comments given above regarding the properties of the various alloying elements also apply to the alloy in accordance with this aspect of the invention.

In a particular preferred embodiment of this aspect of the invention the alloy has a composition by weight percent in the ranges 0.75-0.8 wt % carbon, 1.3-1.4 wt % silicon, 1.9-2.2 wt % manganese, 0.005-0.008 wt % phosphorous, up to 0.005 wt % sulphur, 0.2-0.25 wt % chromium, 0.3-0.35 wt % molybdenum, 1-1.1 wt % nickel, 0.02-0.04 wt % aluminium, up to 0.05 wt % cobalt, up to 0.05% copper, 0.01-0.04 wt % nitrogen, up to 0.001 wt % niobium, up to 0.05 wt % tin and up to 0.01 wt % vanadium, with the remainder (save for incidental impurities) being iron.

The method of making an alloy with either of these compositions will be essentially the same as the method set out in detail above.

A suitable austenitisation temperature for an alloy according to this aspect of the invention will be between 800 degrees Celsius and 1000 degrees Celsius, and preferably between 800 degrees Celsius and 830 degrees Celsius.

A suitable transformation temperature will be between 150 degrees Celsius and 250 degrees Celsius, and preferably between 190 degrees Celsius and 220 degrees Celsius.

If tempering is to be performed, a suitable tempering temperature will be between 250 degrees Celsius and 500 degrees Celsius, and preferably between 330 degrees Celsius and 470 degrees Celsius.

An alloy produced according to this aspect of the invention will typically have an austenite grain diameter of about 10 μm after heat treatment. After transformation, the level of retained austenite is expected to be higher than 10% and preferably within the range 10-20%.

An alloy produced according to this aspect of the invention, with the composition and heat treatment set out above, is expected to have the following mechanical properties:

Tensile strength 2100 MPa at room temperature, and 1700 MPa at 350 degrees Celsius.

0.2% proof stress 1750 MPa at room temperature, and 1350 MPa at 350 degrees Celsius.

Elongation to failure 4% at room temperature and 14% at 350 degrees Celsius.

Hardness 620 HV at room temperature.

Fatigue life (stress range 1400 MPa, R=0) greater than 50000 cycles before failure at room temperature, and greater than 40000 cycles under the same conditions at 350 degrees Celsius.

The alloys according to the invention, with any of the compositions set out above, also exhibit very high thermal stability. Experimental results show that their mechanical properties are retained when the alloy is maintained at a temperature of 300 degrees Celsius for as long as two years.

The invention thus provides a bainitic steel alloy with tensile, fatigue and yield strengths sufficiently high to permit its use for critical components such as shafts in aircraft engines, but without the compromises in mechanical properties that have previously been associated with high strength bainitic steel alloys. 

1. A bainitic steel alloy having the following composition in weight percent: carbon 0.6-1.0; silicon 1-2; manganese 0.5-2.5; phosphorous 0-0.01; sulphur 0-0.008; chromium 0-1.3; molybdenum 0.2-0.4; nickel 0-1.3; aluminium 0.01-0.06; cobalt 0-4; copper 0-0.07; nitrogen 0.005-0.06; niobium 0-0.01; tin 0-0.07; vanadium 0-0.3; the remainder being iron save for incidental impurities.
 2. A bainitic steel alloy having the following composition in weight percent: carbon 0.7-0.85; silicon 1-2; manganese 0.5-2.5; phosphorous 0-0.01; sulphur 0-0.008; chromium 1-2; molybdenum 0.2-0.4; nickel 0-0.08; aluminium 0.01-0.06; cobalt 1.4-5; copper 0-0.1; nitrogen 0.005-0.06; niobium 0-0.005; tin 0-0.1; vanadium 0.1-0.3; the remainder being iron save for incidental impurities.
 3. A bainitic steel alloy as claimed in claim 2, and having the following composition in weight percent: carbon 0.75-0.8; silicon 1.3-1.4; manganese 1.9-2.2; phosphorous 0.005-0.008; sulphur 0-0.005; chromium 1.2-1.25; molybdenum 0.3-0.35; nickel 0-0.06; aluminium 0.02-0.04; cobalt 1.65-1.75; copper 0-0.05; nitrogen 0.01-0.04; niobium 0-0.001; tin 0-0.05; vanadium 0.14-0.16; the remainder being iron save for incidental impurities.
 4. A bainitic steel alloy having the following composition in weight percent: carbon 0.7-0.85; silicon 1.2-2; manganese 0.5-2.5; phosphorous 0-0.01; sulphur 0-0.008; chromium 0-0.35; molybdenum 0.2-0.4; nickel 0.9-2; aluminium 0.01-0.06; cobalt 0-1.5; copper 0-0.1; nitrogen 0.005-0.06; niobium 0-0.005; tin 0-0.1; vanadium 0-0.3; the remainder being iron save for incidental impurities.
 5. A bainitic steel alloy as claimed in claim 4, and having the following composition in weight percent: carbon 0.75-0.8; silicon 1.3-1.4; manganese 1.9-2.2; phosphorous 0.005-0.008; sulphur 0-0.005; chromium 0.2-0.25; molybdenum 0.3-0.35; nickel 1-1.1; aluminium 0.02-0.04; cobalt 0-0.05; copper 0-0.05; nitrogen 0.01-0.04; niobium 0-0.001; tin 0-0.05; vanadium 0-0.01; the remainder being iron save for incidental impurities. 